The exploded view diagrams for these two types are the same as shown below, even though some of the parts are different on the two types.

Source: The Bing Agency

Some Differences Between Type 94 and Type 64 Carburetors

For the R100RS type 94 Bing carburetors, differences include the top cover (20) with only 2 screws (21), internal spring on the slide (22),and the choke return spring (52b).

Type 94 Dome Cover with 2 Screws

Type 94 With Spring Inside Slide

Choke Lever and Return Spring Orientation

Also, the throttle plate adjustment is difference on the type 94 and DOES NOT have screw (36), spring (37), and bracket (31), instead, using screw (34) and the longer bracket (31) with the tabs (27a/b) and (28) with spring (35). Of course, the internal jets are different sizes, but they go in the same place as the jets for the Bing type 64 carburetors.

Throttle Lever, Bracket and Return Spring Orientation

Differences in 1973 R75/5 vs. 1983 R100RS Karcoma Petcocks

There are a couple differences in the Karcoma petcocks. The R100RS has an outlet 90 degrees from the inlet while the R75/5 outlet is direction below the inlet.

R100RS 90 Degree Outlet Petcock

R75/5 180 Degree Karcoma Petcock

The internal filter screens are different; the R100RS is a much longer screen and has the gasket built into it while the R75/5 screen is shorter and has a separate gasket.

Parts List

The following is a list of parts, part numbers and quantities. I had to replace the choke and throttle return springs as these were pretty corroded. I also decided to replace the throttle and choke cables (not shown in the parts list) to avoid surprises that come from unseen corrosion and bent or broken cable strands.

Part #

Description

Qty

16 12 1 234 869

Karcoma, O-ring

2

16 12 1 238 924

Karcoma, Rubber gasket, 5-hole

2

16 12 1 240 060

Karcoma, Fiber gasket, Top

2

16 12 1 233 367

Karcoma, cap petcock

2

13 11 1 336 902

40 mm Carb Rebuild Kit, Pair

1

13 11 1 254 766

Float

2

13 11 1 254 768

Float Hinge Pin

2

13 11 1 335 318

Float Needle

2

13 11 1 335 320

Needle Clip

2

13 11 1 261 702

Needle Jet 2.66 (From 09/1979)

2

13 11 1 335 321

Slide Needle 40mm Carb

2

13 11 1 254 738 SS

Throttle Shaft Screw SS

4

13 11 1 259 870

Vacuum port gasket

2

13 11 1 258 319

Fuel Line, Braided

4

13 11 1 335 312

Throttle Return Spring, 40 mm

2

13 11 1 337 409

Choke Return Spring

2

13 11 1 337 361 SS

Dome Top Screw

4

13 72 1 264 392

Head to Carb Rubber Sleeve, 40 mm

2

Cleaning and Polishing

To restore the finish on the carburetors takes a lot of elbow grease and patience, but the result is worth the time. This section from the R75/5 carbureator rebuild write-up provides the details of what I have found that works.

Here are some of the before pictures from the R100RS type 94 carburetors.

Rusty Screws and Cap on Dome

Grunge on Engine Side

Grunge and Fuel Stains

Rebuild Karcoma Petcocks

Since the rebuild procedure is quite similar between the type 64 used on the R75/5 and the 94 used on the R100RS, and two versions of the Karcoma petcocks, I’m going to reference the write-up I did for the R75/5 and note any differences I found when rebuilding and refinishing the type 94 carburetors and 90 degree outlet Karcoma petcocks.

The design of the R100RS petcock is very similar to the ones used on the R75/5. The procedure I followed is similar to the R75/5 write-up.

First, I drained the gas from tank. I used an upended trash basket to hold the tank aboce a 5 gallon gas can.

Draining Gas Tank

When I unscrewed the petcock from the tank, there was quite a bit of junk in the bottom of the tank that had collected on the strainer screens. The screens were in good condition and doing their job so I did not replace them.

Fuel Screen Is Working

This is the order of the fuel tubes. The short one is for reserve.

Petcock Showing Tube Order

I find that using a philips head screw driver to remove the plastic cap from the petcock works well and reduces the chance of damaging the plastic or your hand.

Using Philips Head Screw Driver to Remove Plastic Cap

You can see the serrations on the nut that holds the petcock handle and internal parts together. The black plastic cap is secured by the serrations.

Plastic Cap Loose from Metal Cap

When I get one side of the cap off, it’s easy to remove the black plastic cap. Then I slide it over the petcock handle to remove it. It just barely slides over the wide part of the handle.

Removing Plastic Cap over Handle

I use a wide,smooth jaw pair of pliers without serrations on the jaw to grip the serrated ring and turn it to remove it from the body of the petcock. You may need to use some Kroil or other rusty nut loosener if you can’t get the ring to budge. Try not to flatten the serrations as these help hold the black plastic cap on.

Pliers on Outside Edge of Threaded Cap

Once the serrated cap is off, the inner spring and parts can be removed. I remove the serrated cap over the handle just as I did the black cap.

Removing Threaded Cap Over Handle

Petcock Insides

New Cap, Rubber Gasket and O-ring

I remove the black rubber 5-hole gasket using a pick. I use the pick to remove the o-ring from the handle and replace them with a new 5-hole gasket and o-ring. I put a bit of silicone grease on the o-ring.

Pick Removing 5-hole Old Washer

I clean and polish the body of the petcock and then reassemble it. Note, the internal plate with the tab aligns in the slot in the petcock body.

Indent in Petcock for Locating Tab

Petcock Handle Locating Tab

Getting the serrated ring to thread into the petcock body is hard due to the strong spring you have to push on while twisting the ring. I align the start of the threads on the ring with the start of the thread in the petcock body so when I twist it the ring thread will immediately engage the thread in the petcock body. I put the petcock body on the floor and then put my weight on my arms to force my fingers down onto the ring and then twist my wrists a quarter turn to engage the ring in the threads. After a couple tries, I get the serrated ring threaded into the petcock body and then use the smooth jaw pliers to tighten the ring.

I install new black plastic covers on the petcocks and then I install the plastic fuel pipes into the body of the petcock and check they are in the correct hole by rotating the petcock handle to “on” and blowing through the long tube to be sure air comes out the outlet. If it doesn’t, I have the tubes reversed.

The petcock nut has a reverse thread on one side. I just start the nut on the petcock body and then turn it on the tank threads while holding the body until it is tight.

Carburetor Disassembly

Due to rusty top cover screws, I soaked them in Kroil for a couple days and then used my impact driver to loosen them. I don’t want to break off the screws and have to try to easy-out the remains.

NOTE: I learned after using the impact driver the way I show below that I was in danger of breaking the top edge of the carburetor casting. In should be supported if you have to use an impact driver to avoid breaking the lip of the casting. I was lucky and didn’t hit the driver very hard to break the screws loose.

Impact Drive to Remove Rusted Top Screws-WRONG WAY TO DO THIS

When I removed the left air tube, a plastic tube was loose inside.

Air Tube with Oil Breather Pipe (Shouldn’t be There)

Oil Breather Tube, Left Side

Further inspection of the air box showed this was one of the two oil breather pipes that direct oil mist into the carburetors. The type 64 on the R75/5 has a single large black plastic pipe that rests inside the right side air tube. The airbox on the 1983 R100RS was redesigned to aid meeting air pollution requirements. The oil breather system is different. The rubber tubing that connects to the plastic pipes inside the air tubes was split, so I ordered new rubber tubes and clips to secure them.

The air tubes also have a metal ring that secures to a rubber gasket inside the airbox.

Air Tube with Metal Ring

Now I removed the hose clamps securing the carburetor to the engine so I could turn it around to remove the throttle and choke cables. The rubber hose is connected to a vacuum port on the carburetor.

Carburetor Ready for Cable Removal

Removing Throttle Cable

Now it’s easy to see the throttle and choke springs on the type 94. These are different compared to the R75/5 type 64 carburetor.

Engine Side of Carburetor Showing Choke and Throttle Springs and Different Throttle Bracket

Remove Float Bowl, Floats and Float Needle

This is straight forward. Remove the bail holding the float bowl then remove the hinge pin securing the float assembly to the carburetor. Then, remove the float needle from the body of the carburetor. The float needle in this type94 is different from the one in the R75/5 type 64 carburetor as it has a rubber tip.

Float Bowl, Very Clean

Damaged Float Bowl Gasket; Affects Idle

Float Bowl Removed, Missing Paint on Emblem

Float Hinge Pin Has Serrations On Right Side, So Push Out To Right

Float Assembly, Float Hinge Pin, Float Needle with Rubber Tip

Remove Throttle Slide Assembly

This is the procedure for the R75/5 type 64 carburetor. It’s the same for the R100RS type 94 carburetor.

Remove Varnish and Carbon

As for the R75/5 project, I soaked the parts in ChemTool B-12 overnight to remove the varnish, carbon and crud. I used some plastic food storage containers which I got at the local supermarket. These are not affected by the ChemTool, WHICH IS VERY NASTY STUFF. I always wear rubber gloves and glasses when working with it. In fact, it will remove the white paint from the black emblems on the side of the carburetors. Since most of the paint was missing anyway, I didn’t bother to protect the emblems. I’ll repaint them later.

I used 000 steel wool on the brass parts to remove any remaining stains and then used metal polish to return their luster. Then I rinsed them to remove and traces of steel wool.

Paint Missing From Badge

Then, I use AutoSol aluminum and metal cleaners to clean the castings and AutoSol metal polish to put a shine back on the parts.

Major Carburetor Castings After ChemTool Soak Before Polishing

Polished Carburetor Bowl and Top – Top Dome Rust Gone

Rebuild Kit Parts

The rebuild kit does not contain all the parts I need. I added new jet needles, needle jets, floats, float hinge pins and float needles.

Full Carburetor Rebuild Parts

Carburetor Rebuild Kit

The R75/5 write-up has a good review of which size o-ring goes with which part.

Assemble Carburetor

Assemble Jets and Mixing Tube

The new o-rings are installed and then the jets and mixing tube are screwed back into the carburetor body. I use some masking tape on the screw threads to protect the o-ring from being cut as they are slide over the body of the jet. By twisting the tape into a taper, it makes it easy to slide the o-ring over the tape. I also use a 90 degree pick to help pull the o-ring until it seats into the groove.

Assemble the Choke

The key is to ensure the left side parts go together and that the choke disk is oriented the correct way in the choke housing as shown in the pictures. The same procedure is used for the right side carburetor ensuring the right side parts go together. For this reason, I assemble one choke at a time to avoid mixing up the left and right side parts.

Choke Housing Gasket – Dry Mounted

Choke Shaft Alignment with Choke Housing

Choke Disk Alignment with Choke Housing

Choke Housing Screws Get Dab of Anti-Seize

Last, I attach the two arms of the choke lever with the nut and washer. It is easy to cross thread the nut and damage the threads in the brass shaft, so be careful.

Inner Choke Lever Plate Orientation

Inner Choke Lever Plate Orientation on Shaft

Outer Choke Lever Plate Orientation

Choke Shaft Washer and Nut

Choke Cable Lever Assemble

Choke Lever with Spring Orientation

Assemble Throttle Shaft, Butterfly and Linkage

I use a dab of silicone grease on the end of the shaft that fits into the boss in the carburetor to prevent corrosion and keep the shaft lightly lubricated. Note, the o-ring goes in the groove farthest from the threads on the throttle shaft. The groove closest to the threads is where the throttle bracket mounts onto the throttle shaft.

Masking Tape for O-ring Installation on Throttle Shaft

Inserting Throttle Shaft with New O-ring Into Carburetor

Throttle Shaft Alignment (Counter Sunk Holes Face Outside)

Sliding Throttle Plate into Throttle Shaft Carefully

Correct Side of Throttle Plate Facing You (3 Punch Marks)

The throttle plate has to align inside the carburetor body while not binding the throttle shaft on the bracket assembly. I align the plate with the holes in the shaft with a pen and then insert the screws with blue locktite until just snug, but don’t tighten them all the way. Then I insert the throttle bracket into the slot on the throttle shaft and make sure the shaft turns freely. I snug the screws for the bracket to the throttle body, but don’t tighten them up. Next, I look through the carburetor and move the shaft and throttle plate as needed until the gap around the plate is more or less uniform and there is a minimum gap all around. Then, I tighten the throttle bracket screws and then the throttle plate screws and check that the throttle shaft still turns freely. If it binds, I loosen the bracket screws, adjust the throttle shaft and throttle plate loosening the throttle plate screws if necessary, and then tight all the screws. The throttle shaft must turn with no binding.

Last, I attach the two arms of the throttle lever with the nut and washer. It is easy to cross thread the nut and damage the threads on the brass shaft, so be careful.

Lining Up Throttle Plate on Throttle Shaft

Carburetor Throttle Plate Screws with Blue Locktite

Adjusting Throttle Shaft Bracket for Correct Throttle Plate Alignment

Throttle Plate Alignment with Minimal Gap All Around the Plate

Throttle Shaft Bracket Tightened Down

Throttle Shaft Bracket and Throttle Lever Assembly

Throttle Shaft Bracket Assembly With Throttle Return Spring

Assemble Throttle Slide and Diaphram

This procedure is the same as for the R75/5 type 64 carburetor found here:

It is important to align the tabs in the rubber diaphragm with the slot in the throttle slide (inner diaphragm tab) and the slot in the top of the carburetor housing (outer diaphragm tab). I like to use a little anti-seize on the screws that secure the diaphragm retaining ring into the throttle side.

Diaphragm Locating Tab for Top of Slide

Diaphragm Retaining Ring

Diaphragm Retaining Ring Screws

Inserting Jet Needle by Pushing and Rotating the Needle (2 Clicks)

Locating Tab on Diaphragm and Slot in Top of Carburetor

Slide with Jet Needle Installed in Main Jet Carrier Assembly

Throttle Slide with Spring

Throttle Slide Spring Installed Under Carburetor Top Dome

I attach the carburetor top with the two screws and a dab of anti-seize. Then attach the choke spring in the hole on the tab in the carburetor top.

Top of Carburetor Monted

Choke Spring and Throttle Spring Attached

Assemble Float and Float Needle

This procedure is the same as for the R75/5 type 64 carburetor found here:

The float needle has a small wire retaining loop that goes through a small hole in the stem of the float needle. The loop goes over the tab on the end of the float assembly.

Float Needle with Metal Loop

Float Needle with Metal Loop Installed

Float Needle with Loop on Float Tab

Float Needle with Loop on Float Tab

The hinge of the float assembly fits between the posts on the bottom of the carburetor and are secured by the float hinge pin. One side of the hinge pin has serrations so push the pin in from the serration side and then gently with needle nose pliers press the hinge pin into the boss of the post. Be careful so you don’t crack or break the post when doing this.

Inserting Float Hing Pin

Use Needle Nose Pliers to Gently Press Hing Pin Into Boss

Now, I adjust the tab on the float so the top of the white float is parallel with the bottom of the groove holding the bowl gasket when the tab just touches the stem on the float needle.

Float Tab on Top of Float Needle Stem

Then I attach the wire bail to the holes in the carburetor body and push the bail over the float bowl to secure it.

Float Bowl Metal Strap In Place

Repainting the White On the Embossed Part of Emblem

The white paint on the emblem was mostly gone when I got the bike and the Chemtool B12 completely removed it. To repaint it, I used a thin piece of molding and put pin stripping paint on one side of the molding and then pressed it evenly on top of the embossed section of the emblem. If I messed up, which happened several times, I just wiped the paint off with lacquer thinner and tried again. After 6 or 7 tries, the embossed part of the emblem has white paint on it again. Where there were a couple small dents in the embossed areas that didn’t fill with paint, I used a very small artists brush and carefully filled them in. Any slight drips or spots on the emblem were easy to remove after the paint tried by carefully using the dull side of an exacto knife blade to scrape them off the black emblem.

Pinstripe Paint and Wood Block

Pressing Wood with Paint on One Side On the Embossed Part of Emblem

Restored White Paint on Embossing

What Done Looks Like

Here is the rebuilt and refinished carburetor and what it looks like mounted on the bike.

Engine Side of Restored / Rebuilt Carburetor

Top Side of Restored / Rebuilt Carburetor

Outlet Side of of Restored / Rebuilt Carburetor

Rebuilt and Refinished Carburetor Mounted on R100RS

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Wonderful to have this information and timely too. My right side carb is leaking a small bit of fuel. With 30K miles on it, I was getting suspicious of the floats. The garbage they call gasoline in the twenty-first century does not help with fuel lines, carburetors, valves, seats or anything else touched by fuel. Looks like you did a great job. I will study your instructions and do my best efforts.
Dale
79 R100RT

The float looks discolored and sticking. Cannot see any gasket damage but that means nothing. Thank you for all the effort and time you have contributed. Are you a member of the Airheads Beemer Club or some other associations, clubs, news letters, etc.?

Gave your list a scan but did not spot what I was looking for so thought I should ask…
Do you have instructions on how to fix valve guides that are causing excessive oil consumption?
Thanks again for this GREAT web site!

I haven’t replaced valve guides myself. I’ve had any head work I needed done (guides, springs, valves, seat grinding) done by Randy Long at Long’s Mechanical Services. In my builds, worn valves and seats required work, so new guides were part of the deal.

I’m pleased this site has been helpful for you. Perhaps one day I’ll dive into rebuilding a head, and if I do, I’ll document what I learn.

I’ve done this Bing rebuild dance many times but always skipped replacing the o rings on the throttle body. Your nice, clear, write up is urging me to have a crack at it. As an owner of an 82 model I will be following your work closely. Thanks in advance!

It’s not very hard to replace those. On older bikes, I worry that air leaks along that shaft will lean out the mixture and make tuning very difficult. One less thing to worry about when trying to get an older machine running well again.

Thanks for coming by. As I state in this write-up:
“I use AutoSol aluminum and metal cleaners to clean the castings and AutoSol metal polish to put a shine back on the parts.”

You can find these at http://www.autosol.com. Specifically I like the Aluminum Cleaner, Multi-Purpose Cleaner (also good on the carburetors as they are made of pot metal not aluminum. Sometimes the Aluminum Cleaner works better and sometimes the Multi-Purpose Cleaner is better) and the liquid Metal Polish. You can find these here.

I just bought my first Airhead as a winter project, and coincidentally it’s an ’83 R100RS. Just stumbled upon your page yesterday. Wow! What do they say about a picture being worth…? What about bunch of pictures? Very detailed pictorial! I know I’ll have my laptop beside me on the bench when I start my carb and petcock rebuilds now! Thanks for taking the time to put this all together and document it so well. Looking forward to your next installments as well.

I’m pleased this material will help you on your project. The 1983 R100RS is running, but needs some more things done which I’ll get to after I finish the rebuild of the 1977 R100RS I picked up in April. I couldn’t resist having the opportunity to work on and ride a first year R100RS 🙂

I recently picked up a 1983 R100RT that had been sold at auction. Supposedly when it was purchased the carbs were off the bike, no battery and has a seeping push rod tube. The guy that bought it threw the carbs on the bike (look very clean like they’ve been rebuilt) but knows nothing about BMW’s. He also thought the oil was coming from the oil pan and he overtightened the pan bolts on that side which then caused it to leak too. I got the bike running before I bought it and rode it home. I ‘ve got a very sooty plug on one side (installed a new plug and no change). I replaced the pan gasket and torqued correctly and changed the oil and filter (oil cooler model). The speedo only goes to 80MPH but from what I’m told its correct for the bike. My next task will be to pull the carbs off and take a look to see if they’ve been rebuilt. Also have a lot of rocker noise on the left side which I believe is the rocker arm walking.

It’s not uncommon for the jet needle to end up in the wrong position and that can cause a rich mixture on one side. Float levels are another likely culprit. I’ve even found different main jets on each side. So, taking it apart to inspect the jets and ensuring they are the correct size, the needles are in the correct position and fuel levels in the float bowls are the same height (for 40 mm carbs, 28 mm fuel height from the bottom of the center well to the top of the fuel).

You are right that rocker blocks not being tight against the rocker arm is the most common source for rocker noise.